1. Technical Field of the Invention
The present invention relates generally to the field of stereophony and, more specifically, to a head related transfer function pseudo-stereophony in which two differentiated audio channels are derived from a single channel.
2. Description of the Prior Art
"Pseudo-stereophony" is a generic description of a family of techniques that allow the derivation of two channels of sound from a monaurally-recorded, one channel source. Signal processing techniques for achieving pseudo-stereophony have existed for at least thirty-six years, a time period roughly paralleling the commercial introduction of home stereo systems. In general, these techniques all involve a process where the two channels of output are derived according to a decorrelation process, i.e., the monaural input is processed to each channel in a differential manner so as to create two non-coherent signals. One technique, known as differential filtering, uses a high-pass filter for output channel one and a low-pass filter for output channel two. Other differential filtering techniques have used two comb filters with complimentary frequency responses, or two all-pass filters with differential phase response.
Another type of pseudo-stereophony can be referred to as reverberation and time delay techniques where, for example, the technique involves adding differentially time delayed, scaled versions of the signal to each output channel, with the possibility of adding recirculated sound.
Yet another type of pseudo-stereophony involves differential sub-audio frequency modulation of the output channels. This is sometimes called "pitch shifting".
The aforementioned techniques suffer from one or more drawbacks, including the overt "coloration" of the sounds timbre in each separate channel of the output. Also, with loudspeaker reproduction the listener is required to sit near the center of the speakers. Further, there is a general inability to mix the pseudo-stereo output to monaural without disturbing the timbral distortions. Also, with stereo reverberation techniques, the problem of additional reverberation time is appreciable. The sound becomes "muddy" (overly reverberant) as a result of convolving an originally reverberant signal with the reverberation of the signal processor (and eventually that of the listening space).
The pinnae of the human ears are shaped to provide a transfer function for received audio signals and thus have a characteristic frequency and phase response for a given angle of incidence of a source to a listener. This characteristic response is convolved with sound that enters the ear and contributes substantially to our ability to listen spatially. This is known as the "head related transfer function" (HRTF). The HRTF was described by Jens Blauret in "The Psychophysics of Human Sound Localization", MIT Press, Cambridge, 1983.
HRTFs have been mentioned in some U.S. patents. For example, U.S. Pat. No. 4,388,494, issued to Schone et al describes a pseudo-stereophonic reproduction circuit utilizing HRTFs. Similarly, U.S. Pat. No. 4,359,605, issued to Haramoto et al describes pseudo-stereophonic signal generation utilizing HRTFs.
U.S. Pat. No. 4,219,696, issued to Kooure et al, discloses stereo signal generation having localized sound images dependent on HRTFs. U.S. Pat. No. 4,192,969 discloses HRTF-related stereophonic signal generation with gauged adjustable attenuators for variable frequency correlation and response.
A continuing need exists for improved pseudo-stereophonic signal generation capable of producing two channel sound derived from a monaural source.